After director Rob Reiner screened the movie for Stephen King, he noticed that King was visibly shaking and wasn't speaking. He left the room and upon his return, told Reiner that the movie was the best adaptation of his work he had ever seen.
In the campfire scene in which Chris breaks down, Rob Reiner was sure River Phoenix could do better. He asked him to think of a time in his own life when an adult had let him down and use it in the scene, which Phoenix did. Upset and crying, he had to be comforted by the director afterwards. The result of Phoenix's exercise is the scene that ended up in the final cut.
Kiefer Sutherland claimed in an interview that in one of the locations of the film, a Renaissance Fair was being held and the cast and crew attended and bought some cookies. Unfortunately, the cookies turned out to be laced with pot and two hours later, the crew found Jerry O'Connell high and crying somewhere in the park.
River Phoenix, Corey Feldman, Wil Wheaton and Jerry O'Connell got into a lot of mischief in the hotel they were staying in during filming. This included throwing all the poolside furniture into the pool, Wheaton fixing video games in the lobby so they could play them for free and Phoenix (spurred on by the other boys) unknowingly covering Kiefer Sutherland's car in mud, only discovering whose car it was when Sutherland confronted a scared and nervous Phoenix about it later.
In an interview by Stephen King in the special features section of the DVD, he reveals that the scene with the leeches, which is straight from his novella, actually did happen to him when he was a child.